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    <title>Document</title>
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    <script>
        function fn(a=100,b=200){
            console.log('a',a)
            console.log('b',b)
            console.log('_________')
        }
        fn()
        // 给a进行赋值，a就使用10
        fn(10)
        // a使用10，b使用20
        fn(10,20)
    </script>
</body>
</html>